The present invention relates to a zoom lens system for use with a 35-mm still camera that features a very high zoom ratio ca. 5 to 6 in consideration of its use with such camera and which is capable of imaging over a broad range of half view angles from the wide-angle end to the telephoto end.
Conventional zoom lens system for use with 35-mm still cameras have generally featured zoom ratios in the range of from 2 to 3. However, the demand of users for higher zoom ratios is still growing today and products that feature zoom ratios of 4 to 6 have come into existence as a result of manufacturing them so as to satisfy specifications that are very stringent in consideration of their use with 35-mm still cameras.
The following products have been proposed as zoom lens systems that are capable of wide-angle viewing for high zoom ratios.
(1) A zoom lens system that attains a zoom ratio of 2-3 and which features a half view angle of approximately 15.degree. at the long focal distance end.
Zoom lens systems in this class are of the two-group type wherein the front group is negative and the rear group is positive, or the four-group type which is a variant of the two-group type wherein the positive rear group is divided into three sub-groups which are positive, negative and positive. Whichever of these types is chosen, zoom lens systems of this first class are basically of the retrofocus type and an increase in the diameter of the last lens is inevitable if an attempt is made to attain zoom ratios of 3 or more or to provide an enhanced telephoto performance.
(2) A zoom lens system that attains a zoom ratio of 3-5 and which features a half view angle of approximately 9.degree. at the long focal distance end.
Unlike those of the first class, zoom lens systems in this class do not use a negative first lens group and consist of either three groups which are positive, negative and positive, four groups which are positive, negative, positive and positive, or five groups which are positive, negative, positive, negative and positive. In the systems of this class, almost all of the lens group are displaced for achieving zooming, though the second, or third or fifth lens group is sometimes designed to be fixed. However, the zoom ratio of up to 5 and the half view angle of 9.degree. that is attainable at the long focal distance end are not completely satisfactory. Also, the half view angle at the wide angle end, for example, 32.degree. is not satisfactory.
(3) A zoom lens system that attains a zoom ratio of more than 5 a half view angle of approximately 38.degree. at the short focal distance end and a half view angle of approximately 9.degree. at the long focal length end.
There are few systems that belong to this class except for those described in Example 1 of Japanese patent application (OPI) No. 161804/1982 (the term "OPI" as used herein means an unexamined published Japanese patent application), Example 2 of Japanese patent application (OPI) No. 161824/1982, Examples 1 to 4 of Japanese patent application (OPI) No. 127908/1983 and in Japanese patent application (OPI) No. 14213/1985. The second lens group of each of the systems shown in these patents is either of the three-unit-five-element or four-unit-five element configuration and probably because of high optical load to be carried by the second lens group, these prior art systems experience great variations in coma abberration and astigmatism during zooming.
(4) A zoom lens system that attains a zoom ratio of 4-5, a half view angle of approximately 38.degree. at the short focal distance end and a half view angle of approximately 9.degree. at the long focal length end.
There are many systems that belong to this class as shown in Unexamined Published Japanese patent application Nos. 4018/1982, 202416/1983, 211117/1983 and 211118/1983. Each one of the systems in this class consists of five lens groups which are positive, negative, positive, negative and positive. In addition, these systems require a complicated mechanism for effecting zooming as illustrated by the use of a special cam device for allowing the first and second lens groups to be displaced ensemble, or the need to displace the third, fourth and fifth lens groups en masse.